Add entry to about:flags for scrollTopLeftInterop

This adds easy UI for developers to test Chrome's new standard-compliant
scrollTop behavior before we make it the default (and for people to
easily check whether turning it off resolves issues shortly after it
becomes the default behavior).

BUG=157855

Review URL: https://codereview.chromium.org/1130073002

Cr-Commit-Position: refs/heads/master@{#328772}
3 files changed